Afform - relationships fill from other entity

Follow-up from https://github.com/civicrm/civicrm-core/pull/23296#issuecomment-1119014194

Let say we want to do a form that allow an employer to update :

• the main contact of the organization
• all the employees

Thanks to #3117 (closed) it's now possible to do a form that will allow to create in one step :

• the organization
• the main contact as Individual 1 with a relationship between the organization and Individual 1
• the employees as Individual 2 with a relationship between the organization and Invividual 2 (which is multiple)

However, there is no way to have an edit mode for such a form. It's possible to add the organization id as an argument but we also need a way to pre-populate the main contacts / list of employees as an option based on the relationships definition.